THE LiFE-HiSTORY OF NUOTJLA fiKLPHINODONTA . 369 



during contraction. A band of muscle occurs near the end 

 of each auricle, that keeps the blood from flowing back into 

 the spaces of the gills. Each auricle is somewhat conical, 

 small where it joins the gill, and considerably enlarged at 

 the end next to the ventricle. The ventricle is swollen at the 

 ends next to the auricles, and flattened over the intestine. 

 The swollen ends of the auricles and the corresponding ends 

 of the ventricle make right and left enlargements that 

 superficially might be mistaken for two hearts. 



A blood-vessel leaves the anterior end of the ventricle on 

 the left side of the intestine, and not in contact with it. 

 Another blood-vessel leaves the posterior end of the ventricle 

 above the intestine and in contact with it. The anterior 

 vessel is somewhat larger than the other. It runs forward 

 over the dorsal end of the stomach and sends branches to 

 the liver and g-enital organs, to the stomach and loops of the 

 intestine, to the foot, to the labial palps, and to the anterior 

 portions of the lobes of the mantle. The vessel that leaves 

 the ventricle posteriorly is at first dorsal to the intestine, but 

 it soon becomes ventral to it, and is distributed to the pos- 

 terior part of the body. 



All of the blood-channels seem to end in rather large 

 connected spaces, that ramify throughout the body. The 

 course of the blood cannot be traced in these spaces. The 

 blood-spaces of the foot, beside providing for the ordinary 

 blood-supply, serve as reservoirs in which blood can be 

 forced to extend the foot. By suppressing some channels 

 and squeezing blood into others different results may be 

 obtained. Blood must undergo respiratory changes in the 

 gills, the mantle lobes, and the palps. 



The opinions of writers on Lamellibranch morphology, 

 regarding the primitive form and position of the heart, are 

 very different. Milne- Edwards (10) thought that the double 

 appearance of the heart of Nucula and Area pointed toward 

 a primitive condition in which the heart was double. Thiele 

 (19), basing his conclusions on Ziegler's observations on the 

 formation of the heart of Cyclas, holds that the heart was 



